Bromate Formation: a Study

In the industrial practice, some seawater is added to distilled water from desalination plant for giving a certain degree of salinity, besides some other adjustments on other chemical physical parameters.

Two samples of raw seawater and distilled water from a waterworks located in the Gulf area were characterized with the following results.

Different mixtures of the two samples were prepared to evaluate the presence of bromate ion in drinkable water.

In order to evaluate if chlorine dioxide leads to formation of bromate ion, samples with 0.3% of seawater and samples with 1.0% of seawater, in amber glassware completely filled and kept closed at a temperature of 40 °C, were treated with two different dosages of chlorine dioxide (0.5 and 1.0 mg/L).

After one hour of contact time the residual concentration of chlorine dioxide was monitored by CPR (chlorine phenol red) method and bromate formation was controlled after flushing with nitrogen for at least 15 minutes.

The data demonstrates that in such conditions chlorine dioxide is stable and does not lead to the formation of bromate ion. One sample (Distilled water + 1.0% Seawater + 1.0 mg/l ClO2) was also kept always at 40 °C for a longer period of time (till 5 days) confirming the previous data.

This laboratory data demonstrates that at 40 °C, into the dark, with extremely long contact time, chlorine dioxide is not able to oxidize bromide even in such condition where the water requires only a small consumption of disinfectant and where there is always a simultaneous presence of chlorine dioxide and the potentially oxidizable bromide in accordance with the electrochemical values of the Redox potentials.

This laboratory data has also been confirmed in the practice with a 5 years disinfection treatment in two waterworks in the Gulf region, where chlorine dioxide has been produced on site with the new ISIA generating system concept, which allows to generate on average 95-98% pure Chlorine Dioxide.

Purity, in particular for drinkable water purposes, is of paramount importance because it is well known that, in some cases, just during the generation process some by-products (chlorate for example) can be formed and then directly inserted into the water to be treated.